The majority of incandescent lamps today use a filament made from tungsten wire which can be of the single or coiled coil design. When initially energized to incandescence, the filament will both metallurgically recrystallize and physically sag under gravitational attraction. Coiled coil filaments, for the most part, have a tendency of sagging more than do single coil filaments and fine wire has a tendency of sagging more than does heavy wire.
In the vertical position, sag is characterized by a collapsing of turns with open turns at the top and compression at the bottom. Sag in the horizontal position is characterized by the formation of one or more catenaries depending on the number of filament support wires.
The preliminary sag in tungsten filaments has never been completely eliminated. However, it can be significantly reduced by utilizing a controlled heating process at the time of initial light-up. Flashing is one particular process known for doing this and is now in common use. Briefly, flashing is a method of stabilizing a filament. It is usually done after the coil is mounted in the lamp and can be performed either before or after tip-off. Since the filament as received is not brittle, it does not require hand mounting and, therefore, can be mounted inexpensively via high speed automatic equipment. Initial light-up under these conditions results in more preliminary sag than on a pre-stabilized coil.
Unfortunately, the filament in an incandescent lamp will continue to sag during subsequent lamp operation in spite of flashing. This is generally attributed to a slippage at the grain boundaries. The condition is known to be aggravated by the presence of oxygen in the vapor state. This accounts for a higher degree of sag in halogen lamps because the halogen regenerative cycle retains a higher percentage of oxygen in the vapor state than there is in a non-halogen incandescent lamp. Generally, the sag in non-halogen incandescent lamps is not severe because most of the residual oxygen is tied up on the bulb wall as tungsten oxide, a colorless solid condensate. Thus, a sufficient quantity of oxygen is not available in the vapor state to promote sagging.
However, in halogen lamps secondary sag and contamination can be a serious problem due to the fact that any oxides present can be reduced by the halogen or halide additive (HBr in this case) which promotes the presence of free oxygen in the vapor state. Also, chemical corrosion of the wire in the cooler sections of the filament results in a significant reduction in life as caused by thinning and premature arcing. This is more pronounced in fine wire than it is in heavy wire.
There are numerous techniques now in use attempting to solve the problem of the existence of contaminants within the halogen lamp and sag of the filament in lamps of this type. Most of these techniques introduce new problems which force a compromise with respect to lamp performance. For the purposes of clarity, some of the more widely used techniques are briefly described here.
Reduced Halogen Content--It has been shown by tests that a reduction in halogen content in the fill gas will give rise to corresponding reduction in filament sag and corrosion. Unfortunately, it will also result in an increase in the percentage of lamps which will turn black prematurely due to failure of the halogen regenerative cycle. Lamp blackening of any halogen lamp constitutes lamp failure even if the filament continues to burn.
Other Halides--The halide additive often used is Hydrogen Bromide (HBr). It is considered by some lamp manufacturers to be too corrosive and, therefore, less desirable than the carbonaceous halides. Tests run fail to show any advantage to using this latter type of halide (CH.sub.2 Br.sub.2, for example). Also, a serious defect arises when using this gas. The result is a significant attenuation of light output which is caused by a carbon layer deposited on the inner bulb wall during initial light-up when the CH.sub.2 Br.sub.2 is decomposed into a more elemental form.
It is believed, therefore, that a tungsten-halogen lamp that provides for means for substantially eliminating contaminants from within the lamp and substantially reduces filament sagging would constitute an advancement in the art.